The Q&A Archives: Oak Tree Borers

Question: I have around 10 Small to Medium sized Oak trees in my yard. They are haveing more and more dead lims every year scattered up and down the tree. I find small wood shavings occationally on the ground around the trunk. Like you have drilled a hole in the trunk. What do you recommed to treat this problem ?

Answer: There are several types of borers which attack oak trees. Oak tree borers are the larvae of moths and beetles which are leaf feeding insects. regardless of the type of borer which attacks your oak trees, they still all borers do the same types of damage and inflict the same sort of wounds to the oak tree.

Damage from oak tree borers manifests as foliage at the top of the tree being sparse and thin looking. The damage from the oak tree borers begins at the top of the trees and migrates downward, with foliage turning yellow or brown and then dying off. As damage continues the individual oak twigs and branches of the oak tree will become weakened due to borer holes. The borer holes are usually in the trunk of the oak tree or on the larger branches. As the borers feed, they leave behind a sawdust like material. Usually the areas of the oak tree which have the largest number of holes and sawdust like material will become the weakest. These branches and their twigs will then begin to die off and the oak tree may lost both foliage and branches. Often the oak tree bark will slough off in areas of extreme populations of oak tree borers. The areas beneath the sloughed off bark will show holes and tunnels filled with sawdust like material. Usually borers attack older branches on trees, but when they infest a younger tree or a newly transplanted one such as a sapling, the oak tree can quickly be killed off entirely if the borer population is large and active.

Once borers have tunneled into your oak tree they can be difficult to control, especially if the insect population is large. You can help to control borer damage by pruning out and destroying all dead or dying branches or those which show signs of tunneling such as sloughed off bark areas. If you have transplants or young saplings which become infested it is best to dig them out and destroy them entirely rather than try to salvage them because these trees will be prime targets for adult females to lay eggs if they make it through to the following season, because of the wounds from the initial infestation. You can also use a lindane containing insecticide in mid to late May on the trunk of the oak tree. This will help control the borer populations on the healthy branches and twigs. The lindane insecticide treatment should be repeated again in two weeks after the first application. You will also want to repeat applications of the insecticide in July and August when new eggs will hatch into young borers and females will be looking for sites to lay eggs. However, the best defense against borer infestations is to keep the oak tree healthy to begin with so that there are no obvious mechanical or insect wounds which will make the oak tree attractive as a place to lay eggs.

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